Intrauterine growth retardation (IUGR) affects 3-8% of human infants and is responsible for as much as 25% of perinatal mortality. Animal models are lacking for this condition. Squirrel monkeys have several characteristics known to be risk factors for human IUGR. This study assesses the relationship between growth measurements and naturally occurring perinatal mortality in squirrel monkeys. Fetal growth and development studies continued in 1996. A standardized technique for ultrasound examination of the pregnant squirrel monkey was developed that can be performed in awake animals in less than 20 minutes. Over 100 perinatal examinations of pregnant dams were performed that included ultrasound imaging and measurements of fetal growth.